Coasting down long grades in neutral will actually cost you MORE money overall because you don’t have any compression braking and you will ride the brakes. Brakes will wear our sooner. Worst-case scenario...brakes will overheat and fade, leaving you with limited stopping power. Happened to me in my Dad’s ‘66 Bonneville with all drum brakes, no discs (brake linings overheated and glazed). Fade is much less likely to occur with four-wheel disc brakes.
Okay, okay, I won’t coast down hill anymore. You all sound like my wife on a nagging Sunday afternoon...and she was right too.
Cars with computer engine management system actually shut off the gas going to the engine while coasting. If you coast in neutral, gas is required to keep the engine running, so along with the other factors you mentioned, you save more on gas by leaving the car in gear.